A backdraft is a rapid or explosive burning of superheated gasses in a fire, caused when oxygen rapidly enters an oxygen-depleted environment; for example, when a window or door to an enclosed space is opened or broken. Backdrafts present a serious threat to firefighters. There is some debate concerning whether backdrafts should be considered a type of flashover (see below).
A backdraft can occur when a compartment fire has little or no ventilation, leading to slowing of gas-phase combustion (due to the lack of oxygen); however, the combustible fuel gases (unburnt fuel vapor and gas-phase combustion intermediates such as hydrocarbons and carbon monoxide) and smoke (primarily particulate matter) remain at a temperature hotter than the auto-ignition temperature of the fuel mixture. If oxygen is then re-introduced to the compartment, e.g. by opening a door or window to a closed room, combustion will restart, often rapidly, as the gases are heated by the combustion and expand rapidly because of the rapidly increasing temperature.
The colour and movement of smoke is used by firefighters to infer fire conditions, including the risk of backdraft. Characteristic warning signs of a backdraft include yellow or brown smoke, smoke which exits small holes in puffs (a sort of breathing effect) and is often found around the edges of doors and windows, and windows which appear brown or black when viewed from the exterior. These darker colors are caused by the presence of large amounts of particulate matter suspended in the air inside the room due to incomplete combustion; it is an indication that the room lacks enough oxygen to permit oxidation of the soot particles. Firefighters often look to see if there is soot on the inside of windows and in any cracks in the window (caused e.g. by the heat). The windows may also have a slight vibration due to varying pressure within the compartment due to intermittent combustion.
If firefighters discover a room pulling air into itself, for example through a crack, they generally evacuate immediately, because this is a strong indication that a backdraft is imminent. Due to pressure differences, puffs of smoke are sometimes drawn back into the enclosed space from which they emanated, which is how the term backdraft originated.
Backdrafts are very dangerous, often surprising even experienced firefighters. The most common tactic used by firefighters to defuse a potential backdraft is to ventilate a room from its highest point, allowing the heat and smoke to escape without igniting.
Common signs of backdraft include a sudden inrush of air upon an opening into a compartment being created, lack of visible signs of flame (fire above its upper flammability limit), "pulsing" smoke plumes from openings and auto-ignition of hot gases at openings where they mix with oxygen in the surrounding air.
Although ISO 13943 defines flashover as "transition to a state of total surface involvement in a fire of combustible materials within an enclosure", a broad definition that embraces several different scenarios, including backdrafts, there is nevertheless considerable disagreement regarding whether or not backdrafts should be properly considered flashovers. The most common use of the term[by whom?] flashover is to describe the near-simultaneous ignition of material caused by heat attaining the autoignition temperature of the combustible material and gases in an enclosure. Flashovers of this type are not backdrafts as they are caused by thermal change. Backdrafts are caused by the introduction of oxygen into an enclosed space with conditions already suitable for ignition, and are thus caused by chemical change.
In the film adaptation of Stephen King's 1408, the protagonist Mike Enslin induces one as a last-ditch effort to kill the room.
The term is also used and is the title of a scene in the 2012 video game Root Double: Before Crime * After Days.
In Bob's Burgers Season 10 Episode "Motor, She Boat", Linda instructs Louise and Gene to say "Backdraft" upon taking a picture of them at the Fire Station Open House. The firefighter on duty informs Linda that backdrafts are actually a very serious problem and not to be taken lightly.